A druid was a member of the educated, professional class among the Celts. The druid class included law-speakers, poets and doctors, among other learned professions, but the best known among the druids were the religious leaders. Very little is known about them because they left no written record but we do know they performed animal and human sacrifice, believed in a form of reincarnation, and held high positions in society. They are portrayed as sorcerers and had mastered the art of shape shifting whereby they could assume the form of any creature.
They were opposed to the practices of Christianity and they played an important part in pagan Celtic society. They were responsible for organizing worship and sacrifices, divination, and judicial procedure and they were exempt from military service and from the payment of taxes. They also had the power to excommunicate people from religious festivals, making them social outcasts. It is claimed that druids were held in such respect that if they intervened between two armies they could stop a battle.
The druids' instruction was secret, and was carried out in caves and forests. Druidic lore consisted of a large number of verses learned by heart, and it took up to twenty years to complete the course of study. All instruction was communicated orally.
A sacrifice acceptable to the gods had to be attended by a druid, for they were the intermediaries between the people and the divinities. The druids predict the future by observing the flight and calls of birds and by the sacrifice of holy animals. In very important matters they prepared a human victim, plunging a dagger into his chest. By observing the way his limbs convulse as he falls and the gushing of his blood, they are able to read the future. They were "administrators" during rituals of human sacrifice, for which criminals were usually used, and that the method was through burning in a wicker man.
A Druid recognized the authority of a single leader, who would rule until his death, when a successor would be chosen by vote or through conflict. They met annually at a sacred place in the region occupied by the Carnute tribe in Gaul, while they viewed Britain as the center of druidic study. Their main teaching was "the souls do not perish, but after death pass from one to another". They were also concerned with "the stars and their movements, the size of the cosmos and the earth, the world of nature, and the powers of deities", indicating they were involved with not only such common aspects of religion as theology and cosmology, but also astronomy.